This invention relates to an improved bearing for use between a railcar truck and the underframe of a railcar body and more particularly is directed to a self leveling bearing having plural sliding friction characteristics responsive to plural load conditions.
The usual railcar assembly comprises a car body supported upon at least one and usually two trucks that, in turn, are carried on axles and wheel sets. The interconnection between truck and car body must permit relative rotation so that the truck may turn as the railcar negotiates curved truck. In the preponderance of freight cars in domestic use the interconnection includes circular center bearing plates and/or bowls mounted centrally of the truck and also transversely centered on the underframe of a car body end. Accordingly, the truck may turn or pivot on the centerplate under the car body and, under certain dynamic conditions and car speeds during operation, the truck may tend to adversely oscillate or "hunt" in a yaw-like manner beneath the car body. Also the car body is subject to adversely roll from side to side during operation. Side bearings are commonly employed to control both such adverse conditions. Heretofore such side bearings have been located on the truck outboard of the center plate and inboard of the wheels to slidingly contact pads secured to the car body underframe. Furthermore, according to a new truck and truck bolster design (disclosed and claimed in a copending U.S. patent application) truck side bearings located outboard of the wheels may be employed to fully support the car and eliminate the need for a center plate.
However, a shortcoming of prior side bearings is that they have offered a single frictional characteristic that is effective for only a limited range of operating conditions. For instance the frictional characteristic has been largely dependent upon the material and size of the bearing surface; but resistance to relative movement also depends on car load and, since car operation is most critical at a fully loaded condition, the side bearings have been designed to permit but adequately dampen sliding movement when the rail car is fully loaded. Unfortunately, this has resulted in inadequate motion damping when the railcar is run empty or under light weight loading. This shortcoming is particularly distressing in the operation of so called "unit" trains which transport a single type of cargo (such as coal) in one direction from a supplier for delivery to a purchaser and then return empty and at high speed in the reverse direction; and where the railcars incorporate light weight construction the damping problem is worsened for empty runs.
Another shortcoming of prior sliding friction type side bearings is that they have been essentially rigidly installed on the truck and/or car body members with the result that the friction face or surface is relatively fixed and non-adjustable with respect to a tilting attitude of the truck and/or car body. This has resulted in the bearing friction face or surface not always being parallel or level with respect to the car body counter-part that is slidingly engaged with consequent loss of effective surface contact.